IN PBODUCINO ASOENDIKG ATMOSPHERIC CURRENTS. 



211 



the desert to the Atlas range, there is little reason to doubt 

 that the air which now goes to Abyssinia might in part flow 

 towards such mountains ; or winds similar to those which 

 now blow from the Gulf of Guinea, might then rush from 

 the Canary Islands, across the now dry deserts to the same 

 elevated land. There would then be not only ascending 

 currents in the air, but rain to water the earth, and if the 

 imaginary mountains were sufliciently lofty, large rivers might 

 flow through the present parched deserts to the sea, as they 

 do now from the Abyssinian and Himalaya mountains. 



But it is only proper that the following passage, which I 

 have seen since the foregoing was written, in Humboldt's 

 Aspects of Nature^ should be given. This eminent philo- 

 sopher says — " It is a remarkable phenomenon, well known 

 among sailors, that in the vicinity of the African coast, between 

 the Canaries and the Cape de Verde Islands, and particularly 

 between Cape Bpjador and the mouth of the Senegal, a west 

 wind often takes the place of the general east or trade wind of 

 the tropics. It is the wide expanse of the Desert of Sahara 

 which causes this westerly wind* The air over the heated 

 sand)' plains becomes rarefied and ascends, the air from the 

 sea rushing in to supply the void so formed, and thus there 

 sometimes arises a west wind adverse to ships bound to the 

 American coast, which are made in this manner to feel the 

 vicinity of the heat-radiating desert, without even seeing the 

 continent to which it belongs. The changes of land and sea 

 breezes which blow alternately at certain hours of the day or 

 night, on all coasts, are due to the same cause." — (vol. i., p. 61.) 

 From this passage it is probable that the eminent writer was 

 so imbued with the idea that air must rise from sun-heated 

 land, as to induce him to refer facts which he found stated to 

 that cause, without making any further inquiry on the subject. 

 But if heated land really produced the effect, should we have 

 only such meagre evidence of it as that given ? The general 



